1. Field of the Invention
The present invention relates to a method for producing butanol using a recombinant microorganism having butanol production capacity, into which genes involved in 2-butanol (or sec-butyl alcohol) biosynthesis have been incorporated, and a recombinant microorganism.
2. Background Art
In recent years, depletion of petroleum resources and global reduction of carbon dioxide emissions have been actively discussed. It is predicted that the petroleum prices will sharply increase in the future. Therefore, development of alternative petroleum materials has been awaited. For example, there have been attempts to bioconvert biomass, sugar, starch, fat and oil, proteins, and the like, which have been produced by plants from water and carbon dioxide, into alternative petroleum materials with the use of solar energy for practical use. An example of such an attempt is the development of the technology of producing plant-derived polylactic acid or polybutylene succinate as alternative plastic materials made from petroleum. Further, ethanol is obtained via fermentative production from sugar, starch, or the like and blended with automobile fuel purified from petroleum so as to be used in the U.S.A., Brazil, and other countries.
In addition, 2-butanol is an important compound that can be used for either fuel or resin material. 2-Butanol is an important substance as a starting material for high-octane automobile fuel or for resin such as propylene. In the past, 2-butanol had been synthesized using petroleum as a starting material. In view of petroleum depletion and the increased amount of CO2 emitted into the air, 2-butanol synthesis via a fermentation process has been desired.
US 2007/0265477 discloses a method for synthesizing 2-butanol from 2-butanone with the use of a carbonyl reductase. WO 2008/098227 discloses a method for synthesizing 2,3-butanediol via a fermentation process and then chemically converting 2,3-butanediol into 2-butanol. Further, WO 2008/137403 discloses a method for directly synthesizing 2-butanol from 2,3-butanediol via a fermentation process.
According to the method disclosed by US 2007/0265477, however, highly purified 2-butanol cannot be obtained because ketones are simultaneously generated. The enzyme reaction of a carbonyl reductase disclosed by US 2007/0265477 requires the use of NADH, which is difficult to mass-produce and is very expensive. A secondary alcohol may be added to the reaction system and NADH resulting from an oxidation reaction thereof may be used. Such method, however, may suffer from the problem of contamination with an alcohol other than the target 2-butanol and an oxide of such alcohol.
The method disclosed by WO 2008/098227 requires the use of a catalyst containing rare metals such as platinum or rubidium at high concentrations (e.g., 5% by weight) in the reaction system. In addition, the method requires the use of a hydrogenation catalyst and an acid catalyst for the entire reaction. According to the method disclosed by WO 2008/098227, the efficiency for chemical conversion into 2-butanol is about 70% at most. That is, this method is disadvantageously low in productivity.
According to the method disclosed by WO 2008/137403, acetolactic acid synthesized from the two pyruvic acid molecules generated in the glycolytic pathway is converted into 2-butanol through acetoin, 2,3-butanediol, and 2-butanone. That is, this method involves many reaction steps and thus is poor in productivity.